Vascular reconstruction is well recognized as a primary means of alleviating tissue ischemia. There are important questions remaining as to the physiologic significance of graft interposition--questions that may be answered by evaluation of graft patency and flow over periods of several years. However, non-invasive assessment of graft flow under physiologic conditions, using methods, currently available, is primarily limited to the study of small superficial vessels or larger vessels more centrally located. Invasive techniques are difficult to perform, require elaborate equipment, are subject to error because of unphysiologic conditions prevalent at the time of the study, usually require hospitalization of the patient and are associated with a degree of morbidity. Permanently implanted telemetry systems have not been widely used in man because of transducer instability, uncertain device longevity, and possible injury to the vessels studied. Therefore, a new method will be evaluated for the postoperative assessment of flow in small vascular grafts. The principle of thermal clearance, using a specially designed probe developed in our laboratory, will be investigated because it will yield flow detection techniques that will be applicable regardless of graft location, structure or electrical impedance. Following initial development and calibration of the apparatus, the invasive studies under physiologic conditions of rest or exercise, a telemetry system will be incorporated. Flow probes and electronic hardware will be constructed in such a fashion as to permit long-term studies so that the technique can be applied to future evaluation of graft function in diverse clinical and research conditions in man and the experimental animal.